Hutton U.S. Pat. No. 739,315 is a typical slit corner mount presently available.
Odgers U.S. Pat. No. 605,007; Blyth U.S. Pat. No. 1,249,713; Cross U.S. Pat. No. 2,517,535; McCabe U.S. Pat. No. 2,637,325; Parker U.S. Pat. No. 4,140,565; Russell U.S. Pat. No. 4,244,128; Edwards U.S. Pat. No. 5,433,023; and Schwartz U.S. Pat. No. 6,874,968 all feature various types of display mounts for photographs and other sheet material.